Stimulation of prothrombinase activity by the nonapeptide Thr-Trp-Ala-Arg-Asn-Ser-Tyr-Asn-Val, a segment of a plant thionin.

Pyrularia thionin (PT) is a basic 47 amino acid peptide isolated from the nuts of Pyrularia pubera. Its structure and properties have been studied in some detail. Its receptor site is a domain of membrane phosphatidyl serine (PS), where it binds with a relatively high specificity. A segment of its covalent structure, the nonapeptide Thr-Trp-Ala-Arg-Asn-Ser-Tyr-Asn-Val, designated serine nonapeptide (SNP), corresponds to amino acids 7-15 of the thionin, except for the position 12 (Ser), which substitutes for Cys, to give stability. This peptide represents what we consider to be the active site of the thionin, and it also binds to PS domains, but less tightly than thionin does. The peptide has an effect on the prothrombinase assay using the chromophore S2238 to measure the thrombin produced by the prothrombinase complex. It is shown that SNP stimulates the prothrombinase complex activity, instead of inhibiting it, as would be expected if it simply covered the PS sites on the membrane of erythrocyte ghosts, used in the prothrombinase assay. SNP appears to substitute for Va in the prothrombinase complex reaction, in a Ca(2+) independent manner, being even more effective in the absence than in the presence of ghosts. In the clotting system, SNP can also substitute for Factor Va.